In general, a battery power supply unit is an electric power source supplying an energy to a related electronic device and a multicell battery pack is applied as the supply unit. By using the multicell pack, rather than a single cell, it is possible to apply a high voltage or to increase a capacity. However, since the cell itself has charge/discharge characteristics, a voltage of each cell tends to be unbalanced as time goes by.
The voltage difference between the cells in the battery pack may generate an un-balancing between the battery cells, thereby causing a capacity loss of the battery pack. With regard to this, it has been developed various battery balancing systems and methods for balancing each cell so as to prevent overcharges of all battery cells and to uniformly charge the cells.
For example, there is a method of allowing a current to flow to a high voltage cell of cells in a battery pack through a resistance and the like so as to adjust a cell balancing. Although this method is simple, it has a disadvantage that a discharge current is increased and heat is much generated if the number of unbalanced high voltage cells is increased. In addition, the method has also a disadvantage that the balance is adjusted to a cell having the lowest voltage of the cells in the battery pack.
In addition, there is a method of allowing a charge current to flow to a low voltage cell of cells in a battery pack so as to adjust a balancing. This method uses a DC-DC converter and has advantages that efficiency is generally high and heat is less generated.
However, the method has also a disadvantage that a cell voltage of the entire battery pack becomes lower than its original lowest voltage if the number of low voltage cells is increased.
Further, there is a considerable difference between read voltages due to a reading timing difference when a voltage of each cell is read. Due to the voltage reading timing difference, the battery balancing system decides that the cells are unbalanced.
It will be described a difference of voltage values according to the voltage reading time of each cell, with reference to FIGS. 1 and 2.
FIG. 1 is a schematic view showing an apparatus for adjusting a voltage balancing using a line selecting device in a lithium ion cell battery according to the prior art and FIG. 2 is a view illustrating a voltage reading timing in a lithium ion cell battery according to the prior art.
Referring to FIG. 1, in the prior voltage balancing adjusting apparatus, when load current flows through a load device 5, terminal voltage of cells are sensitively changed as the load current is varied. The load of the load device 5 is varied as time goes by. Accordingly, when the voltages of the cell terminals are read and compared in a CPU 3 through a line selecting device 1, the voltage value of each cell terminal is differently read even though the voltages of all cells are substantially balanced. As a result, the CPU 3 decides that each of the cells is unbalanced, and thus outputs a balance control signal to control a balance current control section 4.
Referring to FIG. 2, when the cell terminal voltages are read through the line voltage selecting device 1 in the battery balancing system, it can be seen that the cell terminal voltages are not varied if the load current is not changed in the cells (B1, B2, B3, B4). Accordingly, the CPU 3 decides that the voltage of the cell terminal is balanced. However, the terminal voltages of the cells are varied when the load current is changed. Accordingly, when the load current is changed in real-time, the CPU 3 decides that the voltages of the cell terminals are out of balance, due to the voltage reading difference resulting from the load variance as time goes by.
Like this, when the CPU 3 reads the terminal voltages of the cells using the line selecting device 1, the read voltage values of the cells may be different due to the difference of times at which the terminals are selected.
Therefore, it is required a battery balancing system or method capable of eliminating a voltage reading error of cells in a cell system, rapidly performing a voltage balancing and increasing an accuracy of a voltage balancing.
The battery balancing system according to the prior art has diverse condition limitations. For example, it performs a voltage balancing only when the system itself is not operated. Due to the limitations, the voltage balancing operation becomes complicated, an unnecessary time is consumed and an accuracy of the voltage balancing is decreased.